Problem: $-\dfrac{3}{6} + \dfrac{7}{12} = {?}$
$ = - {\dfrac{3 \times 2}{6 \times 2}} + {\dfrac{7 \times 1}{12 \times 1}} $ $ = - {\dfrac{6}{12}} + {\dfrac{7}{12}} $ $ = - \dfrac{{6} + {7}}{12} $ $ = \dfrac{1}{12}$